This invention relates to a method for applying a liquid composition to a structured surface. More particularly, the inventions relates to a method for applying adhesive compositions to a structured release liner.
Pressure sensitive adhesive-backed films may be imaged and adhered to a substrate for decorative purposes. For example, large graphics or smaller decals may be placed on vehicles or may be used as signs for identification, promotional or decorative objectives. However, the tacky and aggressive pressure sensitive adhesives used in these applications cause considerable handling and application problems. Ideally, the film is adhered conformably and evenly on the application substrate. Adhesive-backed articles that adhere with the slightest contact can often be particularly difficult to reapply if they inadvertently adhere to the substrate in an undesired position. In addition, even if one section of the adhesive backed article is properly positioned on a substrate, and the film is firmly adhered, air or other fluids may be trapped under the article. The trapped air forms a bubble in the article, and cannot be easily removed without de-bonding or perforating the article.
The adhesive backed graphic articles described in U.S. Pat. No. 5,897,930, WO 98/29516 and U.S. Ser. No. 09/311,101 (3M File No. 54528USA2A) include an adhesive layer with a plurality of replicated recessed interconnecting microchannels that allow egress of fluids trapped under the article during registration with the substrate. A typical article 10 of this type is shown in FIG. 1, and includes a film 12 having opposed surfaces 14 and 16. The surface 14 of the film 12 is imaged to form a graphic 13. A layer of a pressure sensitive adhesive 18 is bonded to the surface 16 of the film 12. The pressure sensitive adhesive 18 includes a surface 20 that can be bonded to a substrate. The pressure sensitive adhesive 18 includes structures 22 that define a network of channels 24. A release liner 26 is releasably attached to the pressure sensitive adhesive 18. The release liner 26 includes protrusions 28 that form the corresponding channels 24 and structures 22 in the pressure sensitive adhesive 18. The release liner 26, shown in a partially removed state, is fully detachable when pulled in the direction of arrow A and is used to protect the pressure sensitive adhesive prior to application of the article 10 on a substrate.
A portion of a major surface of a liner used in making the article 10 is shown in FIG. 2. The surface of the liner 26 includes a pattern of ridges 28. The surface of the liner 26 includes a series of land areas 23 separated by the substantially continuous ridges 28. The ridges 28 have a substantially hemispherical cross sectional shape.
One convenient method for making the adhesive backed articles 10 includes knife or bar coating solutions of a pressure sensitive adhesive onto a structured surface of a release liner. Referring to FIG. 3, an apparatus 70 is shown that may be used to manufacture the adhesive article 10. A release liner 26 with a structured surface including ridges 28 is fed between a take up roll 72 rotating in the direction of arrow B and a back up roll 74 rotating in the direction of arrow C. A liquid adhesive 75 from a holding tank 76 adheres to the rotating take up roll 72 and is applied to the structured side of the release liner 26. A knife or bar 78 removes a portion of the adhesive 75 and forms a substantially continuous adhesive layer 79 on the release liner 26.
As adhesive is applied to a structured release liner during the manufacture of adhesive backed articles using a knife or bar coating process, the adhesive flows over the structures on the surface of the release liner and fills in the land areas between the structures. In this process coating speed must be controlled to prevent entrapment or agglomeration of air bubbles in the adhesive as it flows into the individual cell-like regions in the land areas between the structures on the release liner. The entrapped air can degrade the performance of the coated adhesive layer and reduce the appearance and printability of the film layer, particularly for high resolution images. These defects detract from the overall appearance of products based on the adhesive backed articles. In addition, the reduced coating speeds are not economical and increase production costs.
The coating process of the invention uses a release liner having features that are mechanically modified to reduce air bubble formation during coating operations and enhance coating speed. The release liners used in the coating process of the invention have structures with geometries designed to reduce air bubble formation during coating and increase coating speed without sacrificing the application, bonding and visual properties of the final graphic film product.
In a first embodiment, the invention is a method for coating including applying an adhesive to a surface, wherein the surface comprises an arrangement of structures thereon. The structures extend upward from a plane of the surface and have at least one sidewall that makes an angle with respect to the plane of the surface of greater than 0xc2x0 and less than 90xc2x0 selected to reduce air bubble formation for a coating solution of a selected rheology. Preferably, the at least one sidewall makes an angle with respect to the plane of the surface of less than about 60xc2x0.
In a second embodiment, the invention is a method for making an adhesive backed article, including applying an adhesive to a release liner, wherein the release liner includes an arrangement of structures thereon. The structures extend upward from a plane of the surface and have at least one sidewall that makes an angle with respect to the plane of the surface of greater than 0xc2x0 and less than 90xc2x0 selected to reduce air bubble formation for a coating solution of a selected rheology.
In a third embodiment, the invention is a method for reducing air entrapment in a coating process, including applying an adhesive to a surface with an arrangement of structures thereon. The structures extend upward from a plane of the surface and have at least one sidewall that makes an angle with respect to the plane of the surface of greater than 0xc2x0 and less than 90xc2x0 selected to reduce air bubble formation for a coating solution of a selected rheology.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.